As computer hardware and software technology continues to progress, the need for larger and faster mass storage devices for storing computer software and data continues to increase. Electronic databases and computer applications such as multimedia applications require large amounts of disk storage space. An axiom in the computer industry is that there is no such thing as enough memory and disk storage space. Mass storage device manufacturers strive to produce high speed hard disk drives with large data capacities at lower and lower costs.
A high speed hard disk drive is one that can store and retrieve data at a fast rate. A disk drive performs write and read operations when storing and retrieving data, respectively. A typical disk drive performs a write operation by transferring data from a host interface to its control circuitry. The control circuitry then stores the data in a local dynamic random access memory (DRAM). A control circuitry processor schedules a series of events to allow the information to be transferred to the disk platters through a write channel. The control circuitry moves the read/write heads to the appropriate track and locates the appropriate sector of the track. Finally, the disk drive control circuitry transfers the data from the DRAM to the located sector of the disk platter through the write channel. A sector generally has a fixed data storage capacity, such as 512 bytes of data per sector. A write clock controls the timing of a write operation in the write channel. The write channel may encode the data so that the data can be more reliably retrieved later.
In a read operation, the appropriate sector to be read is located and data that has been previously written to the disk is read. The read/write head senses the changes in the magnetic flux of the disk platter and generates a corresponding analog read signal. The read channel receives the analog read signal, conditions the signal, and detects "zeros" and "ones" from the signal. The read channel conditions the signal by amplifying the signal to an appropriate level using automatic gain control techniques. The read channel then filters the signal, to eliminate unwanted high frequency noise, equalizes the channel, detects "zeros" and "ones" from the signal, and formats the binary data for the control circuitry. The binary or digital data is then transferred from the read channel to the control circuitry and is stored in the DRAM of the control circuitry. The processor then communicates to the host that data is ready to be transferred. A read clock controls the timing of a read operation in the read channel.
Preamplification circuits in disk drive systems typically include a differential pair of transistors coupled to a high-pass filter circuit. Occasionally, during a read or write operation, the read/write heads may contact the surface of the disk platter, causing the heads to heat up and creating large thermal transient signals. These transient signals may disrupt proper system operation by saturating the amplifiers within the preamplifier circuit. Thermal transient signals are typically comprised of primarily low frequency components. One method of substantially eliminating adverse effects of thermal transient signals is to adjust the bandwidth of high pass filters within the preamplification circuit to reject more low frequency components. This eliminates slow settling low frequency components of the transient signals. High frequency components settle quickly and are, thus, less likely to disrupt system operation.
One approach to modifying the bandwidth of the high-pass filters is to adjust the bias current supplying a differential pair of transistors in the feedback loop. A problem with this approach is that changing the bias current itself creates large, slow settling transient signals. In addition, transistor mismatches often create slow settling transient responses. Problems in dissipating these slow settling transients are exacerbated when the transients are amplified in the subsequent amplification stages that are typically used in conventional designs.
Another approach to modifying the bandwidth of the high-pass filters is to cross-couple the differential pair of transistors in the feedback loop. This approach, however, also suffers from problems associated with slow settling transients caused by mismatched transistors. In addition, this approach results in inefficient power consumption.